Heat shrinkable polymer films have gained substantial acceptance for such uses as the packaging of foodstuffs. This invention pertains to improvements in heat shrinkable films of this nature. The films embodying this invention are normally used as heat shrinkable bags supplied to the food processor with one open end, to be closed and sealed after insertion of the product. After the product is inserted, air is normally evacuated, the open end of the bag is closed, such as by heat sealing, or other means, and finally heat is applied, such as by hot water, to initiate film shrinkage about the product.
Successful shrink bags must satisfy a multiplicity of requirements imposed by both the bag producer and the bag user. Of primary importance to the bag user is the capability of the bag to survive physically intact the processes of being filled, evacuated, sealed closed and heat shrunk. The bag must also be strong enough to survive the material handling involved in moving the contained product along the distribution system.
The bag producer desires a product which can be produced competitively on conventionally available equipment, such as shown in U.S. Pat. No. 4,161,562, while meeting the performance requirements of the user. Thus the bag material should be readily extrudable and susceptible to orientation with sufficient leeway in process parameters as to allow for efficient film production. The orientation temperature should be a temperature which is economically achieved by the producer, and which provides for use of economical shrink processes by the bag user.
Conventional shrink bags have generally been constructed of layers comprising predominantly ethylene vinyl acetate (EVA) copolymers which may or may not be crosslinked, and, in some cases, contain a later of vinylidene chloride copolymer. However, the use of structures containing predominantly EVA may not always be desirable.
As will be appreciated, the processes of stretching the film, and later shrinking it, expose the film to rather severe physical stresses, due to the nature of the operations. In between the stretching and shrinking operations, the film may be subjected to rather harsh tests of its physical capabilities by the various handlings and environments to which it is exposed. It may, for example be shipped in a closed truck exposed to the sun's heat. Thus initiation of the shrink properties must not be triggered at too low a temperature.
In some cases it is desirable to have a shrink bag having lower shrink force than is conventionally experienced without sacrificing the amount of shrink. For example, in packaging blocks of cheese, excessive shrink force yields rounded corners on the soft product. Simultaneously, it is important to retain the conventional amount of shrinkage to ensure film conformity to the product.
In the production of bags, the bags are frequently mounted on adhesively taped rolls; wherein the user removes a bag by forcefully pulling it off the adhesive tape. Thus the interlayer adhesion in the multiple layer bag structure must be sufficient to ensure that the layers of the bag remain adhered to each other during the removal process, helping to provide for the overall internal cohesion of the structure. The bag must have a balance of heat sealability properties, such that the bag can be heat sealed closed over a range of conditions acceptable to commercial operation without excessive faulty seals or burn-throughs. Finally, all these properties must be met in a bag which can be easily laid flat in a shipping container, such as a cardboard box, without undue curling or twisting of the bags.
It is an object of this invention to provide a novel heat shrinkable film which gives improved control of the shrink force while maintaining a high degree of shrink capability.
It is another object to provide such a film which has good heat sealability characteristics and structural cohesion.
It is still another object to provide a process for making a multiple layer tubular film which provides improved control of the shrink process and has good heat sealability characteristics.